I. Field of the Invention
The present invention relates to a composite semipermeable membrane useful for selective separation of liquid mixture, especially for rejecting electrolytes in aqueous solutions with low salt concentration.
II. Description of the Related Art
Composite semipermeable membranes prepared by covering a microporous substrate with a ultra-thin membrane of a cross-linked polyamide obtained by interfacial polycondensation reaction between a polyfunctional amine and a polyfunctional acid halide are now drawing attention as reverse osmosis membranes with high permeability and high separation selectivity. Preparation of the composite membrane by the interfacial polycondensation reaction between the polyfunctional aromatic amine and the polyfunctional acid halide is disclosed in, for example, U.S. Pat. Nos. 4,277,344, 4,520,044, 4,529,646, 4,626,468, 4,643,829, 4,661,254, 4,749,488 and 4,761,234. These composite semipermeable membranes have almost satisfactory high permeability, high selective separation ability and durability even though they are not sufficient from a practical view point. Particularly, their electrolyte rejections from aqueous solutions containing not less than several hundred ppm of electrolytes are extremely high.
However, a problem is brought about when these composite semipermeable membranes are used as a secondary reverse osmosis membrane of two stage reverse osmosis system for producing ultrapure water which is used as washing water of semiconductor devices or the like. One of the purposes of the two stage reverse osmosis system including serially connected two reverse osmosis membrane elements is reducing the load on the ion-exchange process connected after the reverse osmosis system. Since the heaviest load on the ion-exchange tower is carbonic acid, removal of carbonic acid from the raw water is an important purpose of the treatment by the two stage reverse osmosis system. In water, carbonic acid shows equilibrium among carbonate ion, bicarbonate ion and dissolved carbon dioxide gas. According to the equilibrium, under pH of not higher than 6, carbonic acid is mainly in the form of dissolved carbon dioxide gas, under pH of 7-10, carbonic acid is mainly in the form of bicarbonate ion and under pH of not lower than 11, carbonic acid is mainly in the form of carbonate ion. Since carbon dioxide gas dissolved in water cannot be effectively removed by reverse osmosis membrane, it is necessary to make the raw water alkaline so as to convert the carbon dioxide gas to bicarbonate ion or carbonate ion to be effectively rejected by reverse osmosis membrane. Although it is preferred that alkali be added before the first reverse osmosis membrane to most effectively reject carbonic acid, if the raw water contains calcium ion and bicarbonate ion or carbonate ion, since these ions are concentrated by reverse osmosis treatment, insoluble calcium carbonate is formed and deposited on the composite semipermeable membrane, which causes fouling of the membrane surface.
To overcome this problem, it has been proposed to remove a part of the carbon dioxide gas prior to the first stage of the two stage reverse osmosis system (Japanese Laid-Open Patent Application (Kokai) No. 1-231988), or to operate the first stage at a low pH in which calcium carbonate is not precipitated and to operate the second stage at a pH of not lower than 8 by adding an alkali (U.S. Pat. No. 4,574,049, Japanese Laid-Open Patent Application (Kokai) Nos. 62-42787, 62-110795 and 63-28486). A method in which separation membranes with different membrane potentials has also been proposed (Japanese Laid-Open Patent Application (Kokai) No. 61-287492).
However, another problem is brought about if the conventional cross-linked polyamide-based semipermeable membrane is used in such a manner. That is, if the electrolyte concentration is lowered to as low as several ppm, the electrolyte rejection by the composite semipermeable membrane is drastically reduced, especially if the operation of the reverse osmosis is carried out at an alkaline pH.
A high electrolyte rejection is required even for the second stage reverse osmosis treatment wherein the electrolyte concentration is as low as several ppm in order to reduce the load on the ion exchanger connected after the two stage reverse osmosis system. A regenerative ion exchanger tower is usually connected after the reverse osmosis system. Recently, total organic carbon discharged from the regenerative ion exchanger is now drawing attention as a new problem. Thus, it is desired to prepare highly pure water by the two stage reverse osmosis system, which does not require the regenerative ion exchanger. However, for the present, the quality of the water prepared by the two stage reverse osmosis system is not so high such that the regenerative ion exchanger can be omitted. Thus, in order to reduce the number of times of regeneration of the ion exchanger as much as possible, it is necessary to reject the electrolytes of extremely low concentration as low as several ppm, which may be supplied to the second stage of the reverse osmosis system, at a high pH for eliminating dissolved carbon dioxide gas.
To promote the electrolyte rejection by the cross-linked polyamide membrane at a high pH, it has been proposed to coat the membrane with a quaternary ammonium salt (Japanese Laid-Open Patent Application (Kokai) No. 2-2827). However, with this membrane, since the quaternary ammonium salt is only attached to the cross-linked polyamide membrane by electrostatic force, the quaternary ammonium salt is gradually removed from the membrane if the reverse osmosis operation is continued for a long time. Further, the water flux of the membrane is reduced by the quaternary ammonium salt coated on the membrane.
Semipermeable membranes comprising a polymer having quaternary nitrogen atoms are known, which are used mainly as an ion-exchange membrane or ultrafiltration membrane. A reverse osmosis membrane made of a semipermeable membrane of this type is disclosed in Japanese Laid-Open Patent Application (Kokai) No. 63-151303. This reverse osmosis membrane is for treating the waste water from electrodeposition process. Although this reverse osmosis membrane has a relatively good selective rejection performance of cations, the rejection performance of anions and low molecular substances is poor, so that this reverse osmosis membrane does not satisfy the high permeability, high selective separation ability and high durability, which are required for practical reverse osmosis membranes.
On the other hand, the higher the water flux is, the lower the operation pressure can be. Thus, from the economical view point, the reverse osmosis membrane is desired to give a high water flux.